Electrical carrier wave signaling system



March 6, 1945. F. FAIRLEY ELECTRICAL CARRIER WAVE SIGNALING SYSTEM FiledMay 21, 1941 2 Sheets-Sheet 2 AGENT Patented Mar. 6

vided comprising first and second srs'rrm Frank Fairley, London,England, asslgnor to Intel-national Standard Electric New York, N. Y.

Corporation,

Application May 21, i941. Serial No. 394,451 In Great Britain Octobera4, 1940 This invention relates ing systems. I

object of the: invention is to providea to carrier wavesignalsinglechannel system which is intended to have a wider field ofuse than the known \types of single channelsystems hitherto used.According to afeature of the invention, in I a single channel carrier,wave signaling system. transmission is effected over a transmissionlinefrom a first terminal station to a second terminal station on onemodulated sideband of a carrier'frequency and in the other direction onboth modulated side-bands of the same carrier frequency, a source ofcarrier fre quency'waves being provided at the firstterminal stationonly and means being provided" thereat for selecting and demodulatingonly one of the received side bands.

According to another aspect of the invention 'in a carrier wavesignaling system a first terminal station comprises a transmittingbranch arranged to transmit one modulated side-band together withunmodulated carrier frequency waves 'and a receiving branch arranged toselect and demodulate the other side-band of the same carrier frequencyand a'second terminal station comprises. a single circuit which performs the double, function of modulator-and demodulator, the carrierfrequency waves for modulation and demodulation at'the second terminalstation being supplied from the first terminal station. x K

According to a still further aspect of the in:- vention a carrier wavesignaling system is' pro terminal sta tions connected by a' transmissionline and a third terminal station connectedto a point in 4 Claims. (01.178-44) drawings, Figs. 1 and at: which are respectively schematiclay-out diagrams of; the equipment at the two terminal'stations-df asystem according to the invention whilst Fig. 3 is a layout diagram of afurther system-according to the invention.

Referring to Fig. 1, the terminal equipment thereshown. consists of thebasic circuit used ina known type of single channel carrier system.

, Considering the transmission on the carrier channel, speech from theswitchboard is applied via the terminating set of hybrid coil TS to themodulator M, which is of the balanced type, in which the carrier issuppressed. One of the sidebands is selected by the band filter BF andapplied to the transmitting amplifier TA from whence it passes via thedirectional filter said transmission line intermediate said first.

and second terminal stations, 'said' first and second terminal stationseach being arranged to transmit one modulated side band and to the same.carrier frequency, and said third terminal station comprising. a singlecircuit adapted to transmit both modulated side-bands and to receive anddemodulate either side-band transmitted from the' first or secondterminal station, the carrier frequency waves used for modulation ordemodulationat the-,third terminal. station being transmittedover..the'- line from. either one of. the first or second terminal sta-The abovefeatures of the. invention will be better understood from thefollowing descripreceive and demodulate the other side band of tiontaken in conjunction with the accompanying 55.

TDF. and highpass line filter HPL to the open wire or cable line L. Inthe receiving direction the'sideband passes via the receivingdirectional filter RDF to the demodulator DM and the reproduced speechpasses via the low pass filter LP and is amplified by the receivingamplifier RA for transmission to the switchboard via the 4,-wiretermination. The oscillator O supplies the carrier frequency both to themodulator and demodulator. A voice frequency circuit is provided viapthelow pass filter LPL. It may be-arranged by switching that thedirectional filters 'I'DF and RDF are interchanged in circuit positionand that a second hand filter passing the opposite sideband of thecarrier frequency is connected in place of the original band filter,thus converting what may be termed an A terminal into a .B terminal. Theprovision of such a switching arrangement avoids the necessity forhaving available two types of .terminals for any emergency connectionwhich it may be desired to install. This system, since it usessuppressed carrier and a directional transmission of frequencies, may bereadily repeatered and extended to cover any large distances.

The output of the oscillator O isconnected to the' junction of filtersTDF'and RDF.' -The terminal shown in Fig. 1 will then transmit both theupper sideband of the carrier frequency and the carrier frequencyitself. Such a terminal is used in. conjunction with'a'terminal of thetype shown in Fig. 2 which corresponds in its broad aspects with theinert terminal of a, system as described in British specification No.427,605, and requires no power supply for its operation. The uppersideband of the carrier frequency and carrier frequency itself appliedto the inert terminal pass via the line filter HPL to themodulator-demodulator M, which ls an unbalanced type of modulator, andthe reproduced speech passes via the low pass filter LP to theswitchboard. For transmission in the other direction, speech from theswitchboard passes via the low pass filter LP to themodulator-demodulator M, where it modulates the carrier frequencytransmitted from the terminal of Fig. 1. Both sidebands pass via thehigh pass line filter HPL to the terminal of Fig. l, in which the lowersideband only is received via the receiving directional filter RDF. Thelower sideband then is demodulated in the ordinary 'wayby the remainderof the equipment. Such a system will have the advantage of the systemdescribed in British specification No. 427,605 in that no power supplyis required at the inert terminal, but will avoid the limitation of thissystem that the lowest overall transmission equivalent obtainable isthat of the line at the carrier frequency, together with the basiclosses of the two terminals. In this case. due to the use of'onesideband in one direction and to the selection of the other sidebandfrom the two sidebands sent in the other direction, it will be possibleto obtain considerable gain at the Fig. 1 terminal without danger ofsinging, and to make up for the attenuation of the open wire line at thecarrier frequency.

According to a modified arrangement shown in Fig. 3, if two terminalsare located at the two ends of a circuit, one being set to become the Aterminal, i. e. to transmit the upper sideband of the carrier frequencyand the other being set to become the B terminal, i. e. to transmit thelower sideban'd-of the carrier frequency and if it is arranged thateither but not both of these terminals is at the same time allowed totransmit the carrier frequency itself from the oscillator O to line, itis possible to connect in shunt on this circuit at an intermediate pointan inert terminal of the Fig. 2 type and to operate the system using theFig. 2 as an intermediate station. The A" terminal will be similar toFig. 1 whilst the B terminal will also be similar to Fig. 1 with theexception that the connection from the oscillator to the junction of thefilters 'I'DF and RDF will be removed and the filter BF adjusted to passonly the lower side-band. The Fig. 2 terminal may receive thetransmitted sideband from both the "A" or "3 terminal and at the sametime still receive the transmitted carrier from the terminal A which isarranged to transmit this carrier and is thus able to demodulate eithersideband at will. In the other direction, the Fig. 2 terminal generatesboth sidebands one of which is received by the A terminal and the otherof which is received by the B terminal. Since it is possible to arrangethat the carrier is transmitted from either of the terminals "A or "B",it is convenient to arrange that the terminal transmitting the carrieris the nearest one to the Fig. 2 rather" than that the more distant oneshould transmit the carrier. It will be appreciated that in order thatthe system may operate satisfactorily it necessary that the terminal Bwhich is not transmitting the carrier frequency must suppress thecarrierreceived over the line from the distant terminal before demodulation. Ifthere is any appreciable amount of carrier remaining at demodulation,lack of synchronism will cause the usual troubles associafed withexcessive carrier leak on suppressed 1. A single channel two-way carrierwave signaling system comprising a first terminal station, a secondterminal station and a two-wire signal transmission line interconnectingsaid first and second terminal stations, at source of carrier frequencywaves at said first terminal station only, means for transmitting onemodulated side band of said carrier frequency from said first terminalstation to said second terminal station over said transmission line,means for transmitting unmodulated carrier waves from said source oversaid transmission line to said second terminal station, means fortransmitting both modulated side bands of said carrier frequency fromsaid second terminal station to said first terminal station over saidtransmission line, and means at said first terminal station forselecting and demodulating only one of the side bands received thereat.

2. A single channel two-way carrier wave signaling system comprising a.first terminal station, a second terminal station and a two-wire signaltransmission line interconnecting said first and.

second terminal stations, said first station including a source ofcarrier frequency waves, a transmitting branch comprising means fortransmitting one modulated side band of said carrier frequency and areceiving branch comprising means for demodulating the other side bandof the same carrier frequency, and said second terminal stationcomprising a single circuit which performs the double function ofmodulator and demodulator, and means at said first terminal station fortransmitting unmodulated carrier waves from said source over saidtransmission line to said second terminal station for eflectingmodulation and demodulation thereat.

3. A two-way carrier wave signaling system comprising first and secondterminal stations interconnected by a single two-wire signaltransmission line, and a third terminal station connected to a point insaid transmission line intermediate said firstand second terminalstations, means at said first and second terminal stations to transmitone modulated side band and to receive and demodulate the other sideband of thesame carrier frequency, a single circuit at said thirdterminal station for transmitting both modulated side bands of the samecarrier frequency and for receiving and demodulating the side bandstransmitted from the first and second stations, and means at one of saidfirst and second stations for transmitting carrier frequency waves oversaid transmission line to said third terminal station for effectingmodulation and demodulation thereat.

4. A carrier wave signaling system according to claim 2 in which amodulator in said transmitting branch and a demodulator in saidreceiving branch at said first terminal ,station are supplied from saidsource of carrier waves, and an output of said source is also connectedto said transmission line.

